This invention relates to a method and apparatus for measuring temperature, using thermoelectric sensing means, and finds particular application to the measurement of temperatures in inaccessible locations such as, for example, in earth boreholes.
The principle of utilizing a thermoelectric sensing means to detect and measure temperature is well known and is based on the Seebeck effect, namely that in a circuit comprising a pair of junctions formed between dissimilar metals, a voltage will be generated between the junctions when these junctions are at different temperatures, such voltage being a function of the temperature difference and hence for a fixed geometry the heat flow between the junctions. To make practical use of this principle in obtaining accurate temperature measurements, it has been common practice to establish one of the junctions of the circuit as a reference junction, which, by any suitable means, is retained at a known constant temperature, and to expose the other junction to a temperature to be measured, so that the voltage output of the circuit represents the difference in temperature between a known constant temperature and the temperature to be measured, whereby the device can be readily calibrated in relation to the known reference temperature.
Commonly, in temperature measuring systems of the above type, the reference junction is the "cold" junction of the circuit which is maintained at a lower temperature than the temperature to be measured to which the other junction (the "hot" junction) of the circuit is exposed. A cooling medium, with thermostatic or like control, is normally employed to maintain the reference junction at a constant known temperature and heat flows through the system from the hot junction to the cold junction.
Known systems as described above, have certain disadvantages for use in measuring temperatures in applications such as earth boreholes, where for example the device may be required to operate in a hostile environment without maintenance over an extended time period and where read-out of the measured temperature is required at a location remote from the point of measurement. For example, the output signal voltage from conventional thermocouple systems, even multi-junction systems, is relatively low and in practice requires amplification to provide usable information. The system is thus generally complicated by the inclusion of amplification equipment and the problem of low output voltage is accentuated in applications where read-out is to take place at the location remote from the point of temperature measurement, because of line losses. Further, the requirement for maintaining the reference junction at a constant temperature necessiates inclusion in the system of further ancillary apparatus and control means. In the case of an earth borehole for example, this would mean that such ancillary apparatus would have to be included in the equipment which is actually lowered down the borehole and not only are size considerations a factor in this respect, but also the down hole temperatures (possibly approaching 500.degree. C. or more) and general physical conditions may be of an order which precludes practical operation of such ancillary apparatus over extended time periods.
It is an object of the present invention to provide a temperature measuring method and apparatus utilizing a thermoelectric sensing means, which does not rely on maintaining a constant reference junction temperature.
Another object of the invention is to provide a temperature measuring method and apparatus which is particularly suited for use in applications where the point of temperature read-out is required to be situated at a remote location from the point of temperature measurement.
Another object of the invention is to provide a temperature measuring system having a high degree of reliability in performing temperature measurement in a thermally hostile environment over an extended time period.
A further object of the invention is to provide a temperature measuring method and apparatus using thermoelectric sensing means which dispenses with the need for output signal amplification.
Still another object of the invention is to provide a temperature sensing device particularly suitable for use in earth boreholes and like applications.